Int. J. Chem. Sci.: 8(4), 2010, 2763-2774________________________________________*Author for correspondence; E-mail: abohasan_hilla@yahoo.comPHOTODECOLORIZATION OF BISMARCK BROWN R INTHE PRESENCE OF AQUEOUS ZINC OXIDE SUSPENSIONFALAH H. HUSSEIN*, MOHAMMED H. OBIES andABASS A-ALI DREADepartment of Chemistry, College of Science, Babylon University, HILLA (Iraq)ABSTRACTBismarck brown R, (4-[5-C2, 4-diamino-5-methylphenyl) diazenyl-2-methylphenyl] diazenyl-6-methylbenzene-1, 3-diamine dihydrochloride, an anionic azo dye, was degraded photocatalytically underUV irradiation using zinc oxide aqueous suspension. The effects of various parameters, such asphotocatalyst mass, pH of aqueous solution, initial dye concentration, light intensity, the type of currentgas and temperature on photocatalytic degradation were investigated. The percentage of decolorizationwas calculated from the residual concentration by spectrophotometer.The results in this study show that the change in temperature was the fewer factors that effect onthe rate of photocatalytic decolorization. The results indicated that the apparent decolorization efficiencyof bismarck brown R rate was increased slightly with increasing temperature. The activation energy ofphotocatalytic decolorization was calculated and found to be equal to 24 ± 1 kJ mol-1.The results indicate that the rate of decolorization was faster than the total mineralization. Thecomplete decolorization was achieved in less than 60 minutes of irradiation. However, the decrease oftotal organic carbon (TOC) was about 88% after the same period of irradiation.Decolorization and mineralization of bismarck brown R in the absence of light and/or catalystwere performed to demonstrate that the presence of light and catalyst are essential for the decolorization ofthis dye.Key words: Photocatalytic reactions, Bismarck brown R, Zinc oxide, Decolorization efficiency,Mineralization.INTRODUCTIONAzo dyes are used extensively in various industries, such as textile, pharmaceutical,food, cosmetic and printing industries. Thousands of these dyes are used currently and about 2764 F. H. Hussein et al.: Photodecolorization of Bismarck….half million tons are produced annually worldwide1.Grzechulska and Morawski2 reported that the removal of color from wastewaters ismore important than the removal of other organic colorless chemicals. Decolorization ofdyeing factory effluent was regarded very important because of aesthetic and environmentalconcerns3.The illumination of suspended semiconductor in an aqueous solution of dye withunfiltered light (polychromatic light) lead to the possibility of the existence of twopathways4,5.(i) In the first pathway, the part of light with energy equal to or more than the bandgap of the illuminated semiconductor will cause a promotion of an electron to conductionband of the semiconductor and as a result, a positive hole will be created in the valence band.The formed photoholes and photoelectrons can move to the surface of the semiconductor inpresence of light energy. The positive hole will react with adsorbed water molecules on thesurface of semiconductor producing •OH radicals and the electron will react with adsorbedoxygen on the surface. Moreover, they can react with deliquescent oxygen and water insuspended liquid and produce perhydroxyl radicals (HO•2) with high chemical activity6. Theprocesses in this pathway could be summarized in the following equations:Semiconductor + h? ? h+ + e? …(1)h+ + OH? ? •OH …(2)h+ + H2O ? H+ + •OH …(3)e? + O2 ? O2?• …(4)O2?• + H+ ? HO2• …(5)(ii) In the second pathway, the other part of light with energy less than the band gapof the illuminated semiconductor will be absorbed by the adsorbed dye molecules. Dyemolecules will be decolorized by a photosensitization process. The photocatalyticdecolorization of dyes, which is described as a photosensitization processes arecharacterized also by a free radical mechanism. In this process, the adsorbed dyesmolecules(s) on the surface of the semiconductor could absorb a radiation in the visiblerange in addition to the radiation with a short wavelengths7-9. The excited colored dye (S*)(in the singlet or triplet state) will inject an electron to the conduction band of thesemiconductor10. The processes in this path way could be summarized in the followingequations: Int. J. Chem. Sci.: 8(4), 2010 2765S + h? (in the visible or UV regions) ? S* …(6)S* + Semiconductor ? S+• + e? (to the conduction band of semiconductor) …(7)e? + O2 ? O2?• …(8)O2?• + H2O ? OH? + HO•2 …(9)S+• + OH? ? •OH + S …(10)Oliveira et al.11 concluded that the ZnO can be used in the degradation of dyes as analternative to TiO2. They observed that ZnO has higher decolorization velocity than TiO2.Complete decolorization of dyes was achieved after 25 minutes, when ZnO was used while90 minutes are needed to reach the same result, when TiO2 was used. Sakthivela et al.12 alsofound that ZnO can absorb wider spectrum of light than TiO2 can do, when dealing with azodye.The present work, aims to study the photocatalytic decolorization of aqueoussolution of bismarck brown R using ZnO as a photocatalyst. Bismarck brown R, whosestructure is shown in Fig. 1, is a certified biological stain, for microscopy, histology, andcytology, and also used in textile industries.•2HClH N2H C3NH2NNCH3NNH N2 NH2CH3Fig. 1: Structure of bismarck brown REXPERIMENTALZinc oxide with 99.5% purity was supplied by Merck, E. Merck, Darmstadt.Bismarck brown R (standard Fluka for microscopy) was purchased from Fluka (product ofU.S.A.) and used without further purification. Solutions were prepared using distilled water.Photocatalytic decolorization and mineralization processes were carried out in anexperimental setup containing the photoreactor and a gas supply. The gas stream (oxygen ornitrogen) was continuously flowed through the photoreactor. The radiation source was aPhilips mercury lamp (Germany). The radiation source was positioned perpendicularly 2766 F. H. Hussein et al.: Photodecolorization of Bismarck….above the reaction vessel. The suspension of ZnO in 100 mL of aqueous solution ofbismarck brown R was illuminated with UV (A) irradiation at intensity ranging from 1.41 to3.52 mW cm-2. The mean wavelength of ? = 350 nm.In all experiments, the required amount of the ZnO was suspended in 100 mL ofaqueous solution of Bismarck brown R using a magnetic stirrer. At predetermined times; 2mL of reaction mixture was collected and centrifuged for 15 minutes. The supernatant wascarefully removed by a syringe with a long pliable needle and centrifuged again at samespeed and for the same period of time. This second centrifugation was found necessary toremove fine particles of ZnO. After the second centrifugation, the absorbance at certainwavelengths of the supernatants was determined using ultraviolet visible spectrophotometer;type Cary 100 Bio UV-visible spectrophotometer Shimadzu (Varian). Thephotodecolorization percentage of bismarck brown R was followed spectrophotometricallyby a comparison of the absorbance, at specified interval times, with a calibration curveaccomplished by measuring the absorbance, at 230 and 459 nm, with differentconcentrations of the dye solution as shown in Fig. 2. Abs.Wavelength (nm)2.01.51.00.50.0200 300 400 500 600 700 800Fig. 2: UV-Visible spectra of different concentrations of bismarck brown RMineralization of bismarck brown R was assessed by following total organic carbon(TOC) and total inorganic carbon (TIC) at different times of irradiation by using TOC5000A Shimadzu analyzer.pH of the solutions was adjusted with 1 M HCI or 1 M NaOH.Performance efficiency was obtained by using the following equations: Int. J. Chem. Sci.: 8(4), 2010 2767% Degradation efficiency =C C o t ?Cox 100 …(11)% TOC degradation =TOC TOC o t ?TOCox 100 …(12)where, Co and Ct are the initial and final concentration of dye for time t of irradiationand TOCo and TOCt are initial and final total organic carbon of dye for time t of irradiationRESULTS AND DISCUSSIONEffect of catalyst concentrationFig. 3 shows the effect of catalyst concentrations on the decolorization of bismarckbrown R. The rate of decolorization increased with the increasing catalyst concentrationsfrom 1 g L-1 to 3.75 g L-1. Thereafter the rate of decolorization remains constant and then itdecreased with increasing catalyst concentration. These results strongly agreed with ourprevious findings13,14. This behavior could be explained due to increasing total active surfacearea with the increasing catalyst concentration and hence, more active sites on catalystsurface will be available.01020304050607080901000 100 200 300 400 500Decolorization efficiencyAmount of catalyst (mg\100 mL)Fig. 3: Effect of mass of ZnO on decolorization %The increase in catalyst concentration above a maximum level will increase thenumber of particles suspended in the aqueous solution of dye (increasing the turbidity of thesuspension) and as a result, there will be decrease in penetration of irradiation and hence,photoactivated volume of suspension decreases15,16. 2768 F. H. Hussein et al.: Photodecolorization of Bismarck….Effect of pHThe effect of pH on the efficiency of decolorization of 46 ppm of bismarck brown Rwas carried out at different pH ranging between 2-12. The results are given in Fig. 4.Decolorization efficiency was found to depend strongly on pH of solution because thereaction take place on the surface of semiconductor. Fig. 4 shows that the decolorizationefficiency of bismarck brown R increased with increasing pH, exhibiting maximumdecolorization efficiency at pH 9. This behavior could be explained on the basis of zeropoint charge (ZPC)16. The ZPC of ZnO is 9 and with the increase in pH of solution, thesurface of ZnO will become negatively charged by adsorbed hydroxyl ions; However in pHlower than ZPC, the hydroxyl ions adsorbed on the surface will be decreased and as a result,the formation of hydroxyl radicals, which is mainly effective in decolorization process, willdecrease. Fig. 4 shows that the rate of decolorization was decreased dramatically in strongacid media (pH = 2.1). This could be explained due to photocorrosion of ZnO17.00.20.40.60.811.20 10 20 30 40 50 60 70pH=2.1pH=4.5pH=6.7pH=9.0pH=12.0C/CoTime (min)Fig. 4: Effect of pH on decolorization %Effect of dye concentrationThe effect of initial dye concentration on the photocatalytic degradation of bismarckbrown R was studied at different concentrations of dye in the range of (0.2-1.0) x 10-4 M. Fig.5 shows the percent degradation at various initial dye concentrations. It was observed thatthe percent degradation gradually increased with the decreasing initial dye concentration.Percentage of decolorization was found to be 96.2, 92.8, 87.5, 76.9, and 67.5 at (0.2, 0.4, 0.6,0.8 and 1.0) x 10-4 M initial concentrations of dye, respectively. Int. J. Chem. Sci.: 8(4), 2010 2769This behavior may be due to the decrease in the concentration OH? adsorbed oncatalyst surface with the increasing dye concentration. The competitions between OH? ionsto adsorb on active sites of the catalyst will be in the favor of dye ions, when theconcentration of dye was increased13,18. As a result, •OH formation rate decreased and thenthe rate of decolorization also decreases. The inverse proportionality of rate ofdecolorization with dye concentration may be also due to increase of reduction of lightintensity reach the catalyst surface and consequently, photon absorption on surface ofcatalyst is also reduced with the increasing dye concentration13,18.00.10.20.30.40.50.60.70.80.910 10 20 30 40 50 60 70 800.0001M0.00008M0.00006M0.00004M0.00002MC/CoTime (min)Fig. 5: Effect of initial concentration of bismarck brown R onphotodegeradation efficiencyEffect of temperatureReaction was followed at different temperatures in the range 285.15- 301.15 K using350 mg of zinc oxide. The results indicate that the decolorization efficiency of bismarckbrown R with time increases with increasing temperature. Fig. 6 shows that the rate ofdecolorization increases with time at four different temperatures.The acceleration of rate of photocatalytic decolorization of bismarck brown R by arise in temperature may be related to the promotion of the production of free radicals withthe increasing temperature19,20. However, the results indicated that the variation intemperature within the range of 285.15 to 301.15 K does not significantly affect thephotocatalytic degradation of bismarck brown R. These results confirm those presented byprevious authors21-24, where the effect of temperature was explained as the variable with thesmallest effect, especially for values near 323.15 K, where the limiting stage is the 2770 F. H. Hussein et al.: Photodecolorization of Bismarck….adsorption of the dye on the surface of catalyst, but at low temperature, the desorption of theproducts formed limits the reaction because it is slower than the degradation on the surfaceand the adsorption of the reactants on the surface of catalyst23.00.10.20.30.40.50.60.70.80.910 10 20 30 40 50T= 285.15KT= 290.15KT=295.15KT=301.15 KC/CoTime (min)Fig. 6: Effect of temperature on the photodegradation efficiency of bismarck brown RThe activation energy of 24 ± 1 kJ mol-1 for photocatalytic decolorization efficiencyof bismarck R brown was calculated from Fig. 7. The low value of activation energy in this LnK3.3 3.35 3.4 3.45 3.5 3.550-0.5-1-1.5-2-2.5-3-3.51000/TFig. 7: Arrhenius plotwork is similar to our previous findings24-27 for photocatalytic oxidation of different types ofalcohols on anatase and metallized anatase. Kim and Lee23 explained that the very small Int. J. Chem. Sci.: 8(4), 2010 2771activation energy in photocatalytic reactions is the apparent activation energy Ea, whereasthe true activation energy Et is nil. These types of reactions are operating at roomtemperature. The apparent activation energy tends to the heat of adsorption of the productwhereas desorption of the final product from the surface of catalyst is the limiting step.Effect of light intensityThe results listed in Table 1 indicates that the photocatalytic decolorizationefficiency of bismarck R brown increases with increase in light intensity, attaining 100% at2.93 mW cm-2.Table 1: Effect of light intensity on photocatalytic decolorization efficiencyLight intensity(I) (mWcm-2) P.D.E. %0.55 97.71.05 99.21.41 99.61.97 99.922.93 1003.52 100These results are in good agreement with the findings of Lim and Kim28. Theyreported that at light intensity more than one sun equivalent (1-2 mWcm-2, the increase ofrate of reaction is proportional to the square root of light intensity. However, at lightintensity less than one sun equivalent, the increase of rate of reaction is directly proportionalto the light intensity.Mineralization of bismarck brown RThe results shown in Fig. 8 indicate that photocatalytic decolorization of bismarck Rbrown was faster than the decrease of total organic carbon (TOC). The results show that thecomplete decolorization was achieved in less than 60 minutes of irradiation, while thedecrease of total organic carbon (TOC) was about 88% in the same period of irradiation.These findings are in good agreement with those reported before19-20,29. This may be relatedto the formation of some by products, which resist the photocatalytic degradation. 2772 F. H. Hussein et al.: Photodecolorization of Bismarck…. TOC (%)Time (min)Fig. 8: Mineralization of bismarck brown RCONCLUSIONS(i) Control experiments indicated that the presence of UV light, oxygen and zincoxide were essential for the effective destruction of dye.(ii) The photocatalytic decolorization of bismarck brown R using zinc oxide asphotocatalyst strongly depends on the amount of catalyst, concentration of dye,pH, and light intensity.(iii) The temperature is the factor with the smallest effect on the photocatalyticdecolorization of bismarck brown R.(iv) Photocatalytic decolorization of bismarck R brown was faster than the decreaseof total organic carbon (TOC).(v) The photocatalytic decolorization process can expressed by both; the pseudofirst order reaction kinetics and the Langmuir-Hinshelwood kinetic model.ACKNOWLEDGEMENTSThe authors gratefully acknowledge to Prof. Dr. Detlef Bahnemann, "Photocatalysisand Nanotechnology" (Head), Institut fuer Technische Chemie, Gottfried Wilhelm LeibnizUniversitaet, Hannover (Germany) for providing necessary laboratory facilities. 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Muneer, Desalination, 186, 255 (2006).Revised : 24.11.2010 Accepted : 25.11.2010